Radio-Frequency Identification (RFID) systems typically include RFID readers, also known as RFID reader/writers or RFID interrogators, and RFID tags. RFID systems can be used in many ways for locating and identifying objects to which the tags are attached. RFID systems are useful in product-related and service-related industries for tracking objects being processed, inventoried, or handled. In such cases, an RFID tag is usually attached to an individual item, or to its package.
In principle, RFID techniques entail using an RFID reader to interrogate one or more RFID tags. The reader transmitting a Radio Frequency (RF) wave performs the interrogation. The RF wave is typically electromagnetic, at least in the far field. The RF wave can also be predominantly electric or magnetic in the near field. The RF wave may encode one or more commands that instruct the tags to perform one or more actions.
A tag that senses the interrogating RF wave may respond by transmitting back another RF wave. The tag either generates the transmitted back RF wave originally, or by reflecting back a portion of the interrogating RF wave in a process known as backscatter. Backscatter may take place in a number of ways.
The reflected-back RF wave may encode data stored in the tag, such as a number. The response is demodulated and decoded by the reader, which thereby identifies, counts, or otherwise interacts with the associated item. The decoded data can denote a serial number, a price, a date, a destination, other attribute(s), any combination of attributes, and so on. Accordingly, when a reader receives tag data it can learn about the item that hosts the tag and/or about the tag itself.
An RFID tag typically includes an antenna section, a radio section, a power-management section, and frequently a logical section, a memory, or both. In some RFID tags the power-management section included an energy storage device such as a battery. RFID tags with an energy storage device are known as battery-assisted, semi-active, or active tags. Other RFID tags can be powered solely by the RF signal they receive. Such RFID tags do not include an energy storage device and are called passive tags. Of course, even passive tags typically include temporary energy- and data/flag-storage elements such as capacitors or inductors.
At least a portion of the memory is typically implemented as nonvolatile memory (NVM). An NVM comprises one or more memory cells, whose contents may be changed by a write operation. If the NVM employs floating-gate memory cells then the write operation often uses electron tunneling, where a high voltage applied across an oxide surrounding a floating gate induces electrons to tunnel onto or off of the floating gate. Because the physical characteristics of the memory cells may vary due to manufacturing tolerances, oxide thicknesses, etc., electron tunneling often employs a succession of voltage pulses of increasing amplitude, each of which is followed by a data-verification step. The pulses are stopped when the memory cell contains the proper value. This iterative write-verify process writes data to the NVM without prior knowledge of the required tunneling voltage and, at the same time, prevents over-tunneling because each new tunneling pulse ramps to only a slightly higher voltage then the prior pulse. Unfortunately, this approach typically wastes a substantial amount of time by slowly ramping the tunneling voltage from a safe, low value to the required value, where this required value may not be appreciably different from the first time that the memory cell was written.